This invention relates generally to improved apparatus and methods for forming multilayer thermoplastic resin extrusions. More particularly, but not by way of limitation, this invention relates to improved apparatus that includes an injector for dividing a stream of thermoplastic resin and injecting a second stream of thermoplastic resin between the layers of the first thermoplastic resin. It also pertains to an improved method of producing multilayer, thermoplastic resin extrusions wherein a first plasticized resin is formed simultaneously with a second plasticized resin and the first resin is divided into two layers with the second resin being injected between the layers of the first resin.
In the past, a number of different methods and apparatus have been utilized for the purpose of producing multilayer extrusions of thermoplastic resins. In general, the method utilized has been to produce a main stream of base resin, produce a second resin and divide the second resin into two streams which are then placed on the outside of the base resin. Additional layers can be placed on the outside of the first three layers if desired. However, because of the temperatures and pressures involved in the extrusion process and due to the bulk of the apparatus needed to handle the resin layers and such pressures and temperatures, there is a practical limit imposed upon the number of layers that can be placed on the resin in the foregoing manner. Such apparatus and methods are illustrated in copending applications Ser. No. 485,141, filed Apr. 15, 1983, entitled Improved Multi-Layered Sheeting Apparatus and Process Therefor, (now U.S. Pat. No. 4,483,669 issued Nov. 20, 1984.) and Ser. No. 485,550 filed Apr. 15, 1983 and entitled Valve Plate and Feedback Design for Co-extrusion Apparatus and Co-extrusion Process Using Same, (now U.S. Pat. No. 4,483,812 issued Nov. 20, 1984.) both of which are assigned to the assignee of this application.
While the aforedescribed apparatus and methods have worked with a substantial degree of success, it has become desirable to produce multilayer resins wherein five or more layers of various resins are fairly common and up to nine layers of resin are being requested or needed to contain certain products.